Valve make no profit on the consoles themselves right? And if the steam deck profit model of making their money off of games applies to games bought on any handheld, then why not let the other companies have the higher end market and all the research and development costs that making a new console needs. Especially because each new generation would need a refresh to stay on top.(more r and d costs) So why wouldnt valve just keep making their current steam deck that runs stuff well enough for them to keep selling games.
Beyond leaving it to others, the most obvious reasons for not wanting to get into a yearly cycle is that they will have performance increases in mind that they would want to see before it was worth the hassle to transition to a new model and even when they had a sufficient power jump they would still want to deliver battery life that was at least the equal of the current steamdeck.